The short answer to your question is that the beginning of Lent
depends on the date of Easter.

Easter follows a lunar, rather than a solar, calendar and is celebrated
on the Sunday that follows the first full moon after March 21, the vernal
(spring) equinox. Therefore Easter cannot fall earlier than March 22 or
later than April 25.

All the other movable celebrations in the Church calendar ultimately
depend on the date of Easter.

Most of the Eastern Churches follow the same basic principles but often
celebrate Easter on a date different from Catholics and other Western
Christians because they continue to follow the calendar of Julius Caesar
without the corrections incorporated by Pope Gregory XIII in 1582.

Julius Caesar’s calendar calculated the year as 365 days and 6 hours
and thus was about 11 minutes and 9 seconds more than the sun’s actual
course. Although tiny, this excess puts the calendar off by a day, more or
less, every 128 years. Thus, the Council of Nicaea already found it
necessary to regress the date of the spring equinox to March 21 instead of
the original date of March 25.

By the time of Pope Gregory XIII the difference had grown so much that
the spring equinox occurred on March 11.

In 1581 with the bull "Inter Gravissimas" Pope Gregory promulgated a
widespread reform which, among other things, re-established the spring
equinox on March 21 by eliminating 10 days from October 1582. Coincidence
would have it that St. Teresa of Avila died on that very night of Oct.
4-15.

The error of Julius Caesar's calendar was corrected
by deciding that the turn of the century—always a leap year in the Julian
calendar—would be so only when the year could be divided by 400, that is
1600, 2000 2400 2800, etc., whereas there would be no leap year in the
others.

Most Catholic countries, and even some Protestant ones, accepted the
reform almost immediately. Some countries, such as England, held off
accepting the papal reform until 1752 while Russia did not adopt it until
after the Communist takeover in 1918.

The calculation is still not perfect as there is still a difference of
24 seconds between the legal and the solar calendar. However, 3,500 years
will have to pass before another day is added.

Getting back to Lent. This season comprises 40 days before Easter
without counting Sundays which, even though they are called "Sundays of
Lent," are not days of penance. Church tradition has always excluded
fasting and penance on a Sunday.

The tradition of a fast in preparation for Easter goes back to the late
third century but it varied in duration. The tradition of a 40-day fast
was established in Rome between 354 and 384, although it began after the
first Sunday.

As this period was also deemed suitable for the final preparation of
candidates for baptism, the baptismal scrutinies were incorporated with
the rites of this season. Scrutinies are communal prayers celebrated
around the elect to strengthen them to overcome the power of sin in their
lives and to grow in virtue.

Later, at the start of the sixth century, the beginning of Lent was
moved up to Ash Wednesday in order to guarantee 40 days of effective
fasting.